The present invention relates to an output circuit and a battery pack.
The demand for batteries that last longer in portable electronic equipment, such as notebook computers, has grown in recent years. The life of such a battery can be prolonged by increasing the capacity of the battery and decreasing power consumption in each circuit. Thus, decreasing the power consumed by output circuits of portable electronic equipment will help to prolong the life of the battery.
A notebook computer is provided with a rechargeable battery pack. The notebook computer may also be provided with a backup battery pack, which has a large capacity and can be used for a long period of time. A typical battery pack includes an output circuit for generating a control signal used to indicate whether power can be supplied properly. The output circuit generates the control signal when the battery pack is electrically connected to the computer.
A conventional output circuit employs a bipolar transistor to form an open collector circuit. An output circuit employing a MOSFET, which output loss is small, to form an open drain circuit has also been proposed. The drain of the MOSFET sends control signals to circuits in the computer that are concerned with the processing of electric power.
The battery pack may be disconnected from the computer. When the battery pack is disconnected and then reconnected to the computer, the circuits or semiconductor devices within the computer must be activated immediately. Thus, the MOSFET is always in an on state.
FIG. 10 shows an example of a prior art output circuit, which includes a p-channel output MOSFET 51. The source of the MOSFET 51 is connected to a power supply line via a current detecting resistor Rs. A lithium ion battery (not shown) incorporated in the battery pack supplies a power supply voltage Vdd to the power supply line. The drain of the MOSFET 51 is connected to an external output terminal of the battery pack. The gate of the MOSFET 51 is connected to the collector of a drive bipolar transistor 52 via a resistor R2, which forms a bias circuit with a resistor R1. The power supply line, which provides the power supply voltage Vdd, is connected to one end of the resistor R1.
The bipolar transistor 52 goes on when a high drive control signal SG1 is input to the base of the bipolar transistor 52 via a resistor R3. This decreases the gate voltage of the MOSFET 51 to the voltage determined by the bias circuit (formed by the resistors R1, R2) and causes the MOSFET 51 to go on. An output voltage Vout, or control signal, is output from an external output terminal and sent to the semiconductor devices in the computer that are related with the processing of power.
The bipolar transistor 52 goes off when the drive control signal SG1 falls. This increases the gate voltage of the MOSFET 51 to the power supply voltage Vdd and causes the MOSFET 51 to go off.
The current detecting resistor Rs is connected between the source of the MOSFET 51 and the power supply line. The source of the MOSFET 51 is connected to the base of a current restricting bipolar transistor 53. The collector of the bipolar transistor 53 is connected to the gate of the MOSFET 51. The emitter of the bipolar transistor 53 is connected to the power supply line.
When the MOSFET 51 is on, a large output current Iout flows into the MOSFET 51 if a short circuit occurs between the external output terminals for some reason. This increases the voltage between the terminals of the current detecting resistor Rs (Iout.times.Rs). More specifically, the base emitter voltage Vbe of the current restricting bipolar transistor 53 increases and the collector current of the bipolar transistor 53 flows into the resistor R2 of the bias circuit. This increases the gate voltage of the MOSFET 51, or decreases the voltage between the gate and source of the MOSFET 51, and causes the MOSFET 51 to go on, thereby suppressing the increase of the output current Iout.
However, current constantly flows in the output circuit because the bipolar transistor 52 is on when the MOSFET 51 is on. In other words, current flows through the bipolar transistor 52 since the MOSFET 51 is on, even if the battery pack is detached from the computer, when there is no short circuit. Thus, current is consumed by the bipolar transistor 52, which drives the MOSFET 51, even when the battery pack is not being used.